The present invention relates to cameras.
More particularly, the present invention relates to single lens reflex cameras provided with circuits for automatically determining the exposure time.
Cameras of this type normally include an internal light-receiving element for measuring the received light. Because the light is directed to the light-measuring means by a swingable mirror, which prevents light from reaching the film, this mirror must be swung up to an exposure position just prior to exposure so that the light can reach the film, and as a result the light-measuring means no longer receives light. Therefore it is necessary to retain an electrical quantity corresponding to the light which is measured, and for this purpose it is conventional to utilize a memory capacitor connected to the light-measuring means through a switch, the latter switch being automatically opened in connection with upward swinging of the mirror so that the memory capacitor will then retain the electrical quantity corresponding to the measured light. Of course, it is also possible to interrupt the travel of light to the light-measuring means by displacing the latter out of the light path, but this arrangement also necessitates memorizing an electrical quantity corresponding to the light input information at the time immediately prior to opening of the shutter until the time when the shutter starts closing.
As is well known, photographs are made under conditions which vary over a wide range. Under certain undesirable conditions the object which is to be photographed is situated in an environment where a bright light is located behind the object to be photographed, known as back-lighting or counter light, or it may be desired to photograph an object which is illuminated by spot light so that the field in which object is located is dark, with the object itself being brightly illuminated by the spot light. However, there are other conditions where for special effects or for any other purpose the photographer may wish intentionally to change the exposure which otherwise would be automatically determined. Thus, it is recognized that there are conditions where it is necessary to correct or at least change the exposure which otherwise would be provided by the automatic circuitry of the camera.
For the latter purposes a memory device such as the above memory capacitor can be conveniently used. For example when photographing a back-lighted object a camera provided with a memory capacitor of the above type can first be directed to a relatively dark area, and a manual memory switch can be set by the operator in a memory mode in order to memorize the light input from the relatively dark area. Then it is possible to provide an exposure obtained on the basis of the memorized light input when the camera is directed to the object which is situated in front of a brightly illuminated field, provided that the diaphragm aperture size and film speed are maintained unchanged. Thus, under these conditions an exposure correction can be carried out.
However, in order for the memory capacitor to operate it is necessary that the circuit in which it is located is energized, and for this purpose such a circuit has a power switch which must be closed so that the circuitry will render the capacitor operative to receive the electrical quantity which is to be memorized. In general, automatic electric circuits for controlling a camera shutter are battery powered, so that in order to save the energy of the battery and give the latter the longest possible operating life, the power switch for rendering the circuit operative is closed for the minimum amount of time. Normally when the manually operable shutter-tripping plunger is depressed, the initial part of the movement thereof will close the power switch so that the entire automatic circuitry is only turned on just prior to actual exposure in response to the movement of the shutter-tripping plunger through its initial increment of movement.
With such a camera which includes such a power switch, it is essential that the manual memory operations be carried out after the power switch is closed by depressing the shutter-tripping plunger at least partially. These operations are extremely delicate and complicated so that erroneous operation easily results under these conditions.
An additional example of conditions where it is desired to open the circuit to the memory capacitor manually is in connection with the use of single lens reflex cameras for microscopic or copying photography. Single lens reflex cameras are highly effective for purposes of this latter type because of the unique devices which are incorporated into single lens reflex cameras. However, it is extremely important when making photographs of this latter type to eliminate any blur in the resulting photograph, and from this standpoint a single lens reflex camera is disadvantageous in that when the mirror is automatically swung to its exposure position, an instant before actual exposure of the film unavoidable vibrations are created in the camera with the danger of blurring of the exposure which is received on the film.
Of course, this latter problem has been recognized. Conventionally in order to avoid this drawback a camera is provided with a manually operable structure for manually placing the mirror in its exposure position where it is swung up from its normal position inclined downwardly across the optical axis. The manual placing of the mirror in its upper exposure position is also conventional when utilizing a lens system having a short focal length.
However, while it is indeed possible for an operator to place the mirror manually in its upper exposure position, this operation is achieved at the sacrifice of automatic determination of the exposure time. Thus, with cameras which automatically determine exposure time it is essential that the light which is reflected from the mirror be measured in order to determine the exposure time. This measuring of the light reflected from the mirror must take place immediately before exposure when the mirror is swung up and the diaphragm is stopped down so as to provide an automatic accurate determination of exposure time. Under conditions where the mirror is manually swung up such automatic exposure controls cannot be achieved because the light can no longer reach the light-measuring means which provides the electrical quantity which is used as one of the factors in determining the exposure time.